elicited, it results in cessation of inspiration by stimulating the off-switch neurons in the medulla. This reflex is inactive during quiet breathing and appears to be most important in newborns. Stimulation of nasal or facial receptors with cold water initiates the diving reflex. When this reflex is elicited, apnea, or cessation of breathing, and bradycardia occur. This reflex protects individuals from aspirating water in the initial stages of drowning. Activation of receptors in the nose is responsible for the sneeze reflex.
The aspiration or sniff reflex can be elicited by stimulation of mechanical receptors in the nasopharynx and pharynx. This is a strong, short-duration inspiratory effort that brings material from the nasopharynx to the pharynx, where it can be swallowed or expectorated. The mechanical receptors responsible for the sniff reflex are also important in swallowing by inhibiting respiration and causing laryngeal closure. For anatomical reasons, only newborns can breathe and swallow simultaneously, which allows more rapid ingestion of nutrients.
The larynx contains both superficial and deep receptors. Activation of the superficial receptors results in apnea, cough, and expiratory movements that protect the lower respiratory tract from aspirating foreign material. The deep receptors are located in the skeletal muscles of the larynx, and they control muscle fiber activation, as in other skeletal muscles.
Three major types of sensory receptors located in the tracheobronchial tree respond to a variety of different stimuli, and those responses result in changes in the lung’s mechanical properties, alterations in the respiratory pattern, and the development of respiratory symptoms. Inhaled dust, noxious gases, and cigarette smoke stimulate irritant receptors in the trachea and large airways that transmit information through myelinated vagal afferent fibers. Stimulation of these receptors results in an increase in airway resistance, reflex apnea, and coughing. These receptors are also known as rapidly adapting pulmonary stretch receptors. Slowly adapting pulmonary stretch receptors respond to mechanical stimulation, and they are activated by lung inflation. They also transmit information through myelinated, vagal afferent fibers. The increase in lung volume in people with COPD stimulates these pulmonary stretch receptors and delays the onset of the next inspiratory effort. This explains the long, slow expiratory effort in affected individuals, and it is essential to minimize dynamic, expiratory airway compression.
In addition, specialized sensory receptors located in the lung parenchyma respond to chemical or mechanical stimulation in the lung interstitium. These receptors are called juxta-alveolar, (or J) receptors. They transmit their afferent input through unmyelinated, vagal C fibers. They may be responsible for the sensation of dyspnea (abnormal shortness of breath) and the rapid, shallow ventilatory patterns that occur in interstitial lung edema and some inflammatory lung states.
VO2 (L/min) . 1 1.5 2 VCO2 . PaCO2 pH Ventilation PaO2 Lactate •Fig. 25.8 The Two Main Categories of Sleep Apnea. A, Obstruc-tivesleepapnea,thepleuralpressureoscillationsincreaseasCO2levelrises.Thisindicatesthatresistancetoairflowisveryhighasaresultofupperairwayobstruction.B, Centralsleepapneaischaracterizedbynoattempttobreathe,asdemonstratedbynooscillationsinpleuralpressure.
•Fig. 25.7 Oxygen Consumption (V̇ O2) as a Function of the Metabolic Changes That Occur During Exercise. Theanaerobicthreshold(arrow) isthepointatwhichtheillustratedvariableschangeandisduetolacticacidosis.PaCO2,partialpressureofarterialcarbondioxide;PaO2,partialpressureofarterialoxygen;VCO2,carbondioxide consumption.